1. Field of the Invention
The present invention relates to a projection system in which imaging light emitted from a projector is projected on a projection screen to display thereon an image. More particularly, the present invention relates to a projection screen capable of sharply displaying an image and of providing high visibility, and to a projection system containing such a projection screen.
2. Description of Related Art
A conventional projection system usually operates as follows: imaging light emitted from a projector is projected on a projection screen, and viewers observe the light reflected from the projection screen as an image.
Typical examples of projection screens for use in such conventional projection systems include white-colored paper or cloth materials, and plastic films coated with inks that scatter white light. High-quality projection screens that comprise scattering layers containing beads, pearlescent pigments, or the like, capable of controlling the scattering of imaging light, also are now commercially available.
Projectors have become smaller in size and moderate in price in recent years, so that not only demand for projectors for commercial use but also demand for household projectors such as projectors for family theaters is growing, and an increasing number of families are now enjoying projection systems. Household projection systems are often placed in living rooms or the like, which are usually so designed that environmental light such as sunlight and light from lighting fixtures is abundant. Therefore, projection screens for use in household projection systems are expected to show good image display performance even under bright environmental light.
However, the above-described conventional projection screens cannot show good image display performance under bright environmental light because the screens reflect not only imaging light but also environmental light such as sunlight and light from lighting fixtures.
In such a conventional projection system, differences in the intensity of light (imaging light) projected on a projection screen from a projector cause light and shade to form an image. For example, in the case where a white image on a black background is projected, the projected-light-striking part of the projection screen becomes white and the other part becomes black; thus, differences in brightness between white and black cause light and shade to form the desired image. In this case, in order to attain excellent image display, it is necessary to make the contrast between the white- and black-indication parts greater by making the white-indication part lighter and the black-indication part darker.
However, since the above-described conventional projection screen reflects both imaging light and environmental light such as sunlight and light from light fixtures without distinction, both the white- and black-indication parts get lighter, and differences in brightness between white and black are decreased. For this reason, the conventional projection screen cannot satisfactorily provide good image display unless the influence of environmental light such as sunlight and light from lighting fixtures on the projection screen is suppressed by using a means for shading a room, or by placing the projection screen in a dark environment.
Under these circumstances, studies have been made on projection screens capable of showing good image display performance even under bright environmental light. There have so far been proposed projection screens utilizing holograms, polarized-light-separating layers, or the like (see Japanese Laid-Open Patent Publication No. 107660/1993 (JP '660) and Japanese Laid-Open Patent Publication No. 540445/2002 (JP '445)).
Of these conventional projection screens, those using holograms have the advantage that the white-indication part can be made lighter if the scattering of light is properly controlled, so that the screens can show relatively good image display performance even under bright environment light. However, holograms have wavelength selectivity but no polarization selectivity, meaning that the projection screens using holograms can display images only with limited sharpness. Moreover, it is difficult to produce large-sized projection screens by utilizing holograms due to production problems.
On the other hand, using the above-described conventional projection screens with polarized-light-separating layers, it is possible to make the white-indication part lighter and the black-indication part darker. Therefore, these projection screens can sharply display images even under bright environmental light as compared with the projection screens using holograms.
Specifically, for example, JP '660 describes a projection screen having a cholesteric liquid crystal that reflects red, green and blue light (right- or left-handed circularly polarized light) contained in imaging light. This projection screen is made not to reflect nearly half the environmental light incident on the screen by making use of the circularly-polarized-light-separating property of the cholesteric liquid crystal.
However, in the projection screen described in JP '660, since the cholesteric liquid crystal is in the state of planar orientation, specular reflection occurs when the projection screen reflects light, and the reflected light cannot be well recognized as an image. Namely, to recognize the reflected light as an image, it is necessary that the reflected light be scattered. However, JP '660 is silent on this point.
On the other hand, JP '445 describes a projection screen using, as a reflective polarization element, a multi-layered reflective polarizer or the like, having diffusing power. This projection screen does not reflect part of the environmental light incident on the screen because of the polarized-light-separating property of the multi-layered reflective polarizer, and diffuse-reflects the rest of the incident light due to interfacial reflection that occurs in the multi-layered reflective polarizer composed of materials having different refractive indices, or by means of a diffusing element provided separately from the multi-layered reflective polarizer. JP '445 also describes a projection screen using in combination a cholesteric reflective polarizer as a reflective polarization element and a diffusing element. This projection screen does not reflect part of the environmental light incident on the screen because of the polarized-light-separating property of the cholesteric reflective polarizer, and diffuse-reflects the rest of the incident light by means of the diffusing element provided separately from the cholesteric reflective polarizer.
Of the projection screens described in JP '445, the former one must contain a multi-layered reflective polarizer or the like that is a linear polarization element (“DBEF” manufactured by 3M Corporation, etc.). When this projection screen is incorporated into a projection system or the like, it is necessary to make the plane of polarization of the linear polarization element agree with the plane of polarization of a projector that emits linearly polarized light, such as a liquid crystal projector. If these planes of polarization do not agree with each other, excellent image display cannot be attained.
Further, of the projection screens described in JP '445 the latter one contains, as the reflective polarization element, a circular polarization element such as a cholesteric reflective polarizer. However, since the diffusing element for scattering the reflected light is provided on the viewer's side of the reflective polarization element, the polarized-light-separating property of the reflective polarization element is impaired, and image visibility cannot be fully improved.
Namely, since the diffusing element is provided on the viewer's side of the reflective polarization element, light passes through the diffusing element before entering the reflective polarization element, and its state of polarization is disturbed, which is called “depolarization.” Light that passes through the diffusing element includes two types of light, that is, environmental light (sunlight, etc.) and imaging light. If the state of polarization of environmental light is disturbed by the diffusing element, the light that the reflective polarization element inherently transmits is, because of depolarization, converted into a component that the reflective polarization element reflects, and this component is reflected from the reflective polarization element as unnecessary light. On the other hand, if the state of polarization of imaging light is disturbed by the diffusing element, the light that the reflective polarization element inherently transmits is, because of depolarization, converted into a component that the reflective polarization element does not reflect, and this component passes through the reflective polarization element. Because of these two phenomena, the original polarized-light-separating property is impaired, and image visibility cannot be fully improved.
Moreover, in the projection screens described in JP '660 and JP '445, it is necessary to provide anti-glaring layers in order to prevent the projection screens from glaring. The polarized-light-separating property is impaired also by such anti-glaring layers.
In sum, the above-described conventional projection screens, including those ones using holograms and those ones described in JP '660 and JP '445, using polarized-light-separating layers, can display images only with limited sharpness under bright environmental light. Therefore, it has so far been impossible to fully improve image visibility.